11.2.4. Field studies

The optimal area for tracking bees with the harmonic
radar is an open, flat and horizontal landscape without any radar reflecting
obstacles (trees, houses). The reason for these requirements is that the radar
beam should not be reflected either from the ground or from an object because
the reflected radar pulse with its energy multiple times stronger than the
second harmonic signal radiated off from the transponder would interfere with
this small signal. In such an ideal area, transponder-carrying bees can be
detected within a radius of up to 3 km (see section?). Examples are given in
Fig. 25. The transponder is detected every 3 s defined by the revolution
of the radar unit. The circular map resulting from the radar scans is
transformed into a Cartesian map with a custom-written program. Bees travelling
with 5 – 20 km/h are therefore detected about every 5 – 15 m. The length and
width of the radar paint (the technical term for a radar signal) depends strongly
on the strength of the harmonic signal, but usually lies in the range of about
15 m at a distance of 600 m. Additional processing of the radar signals allows
to substantially improve spatial/temporal resolution. Signal strength of the
harmonic radar depends also on the animal's flight height over ground. The unit
described above allows for detecting bees reliably between 70 cm and 9 m height
for distances up to 1,200 m straight away from the radar. Bees learn quickly to
fly with the transponder. Usually, they first land after being released but
then start and fly equally fast and reliably as those without a transponder.
However, they appear to be more sensitive to wind speeds above > 20 km/h. At
such wind speeds they also fly low, making it more difficult to detect them.

The
harmonic radar system allowed proving von Frisch's proposal (von Frisch, 1967)
that bees communicate distance and direction in their waggle dances (Riley et al., 2005; see section 9). In
addition, their navigation strategies could be characterized (Menzel et al., 2005, 2011b). The experiments
performed led to the conclusion that navigating bees integrate multiple
learning strategies which provide them with the capacity to localize themselves
and several goals in such a way that they can perform novel short-cutting
flights between them (Menzel et al.,
2011a). Such a capacity has been related to a memory structure best
conceptualized as a cognitive map (Tolman, 1948).

Fig. 25. Nine examples of flight tracks. In the subfigures A – F three phases of the
homing flights are marked in colour. Vector flight component in red, search
flight component in blue, straight homing component in green. H marks the
location of the hive, R2 and R9 two different release sites. These bees
followed a waggle dance that indicated a location 200 m to the east. The
thin blue line in C marks a portion of the flight which could not be detected
most likely because the bee flew very close to the ground. G – I: Each of these three
bees was first trained to a location indicated as FT30° or FT60°. Then feeding
at this site was terminated, motivating the bees to stay inside the hive and
attend dances. A single dancing bee indicated a location FD30'' or FD60°. Each
bee flew first to the place indicated by the dance and then flew directly to
the learned place along a novel shortcut. The green area around the place
indicated by the dance marks the visual catchment area as defined by the person
sitting there and the spatial resolution of the bee eye. Since the spatial
resolution of the bee eye is 1.5° it can be calculated at which distance the
bee will see the person sitting in front of the hive. There was no visual mark
at the trained place during the test (after Menzel et al., 2011b).